The present invention relates to a photoelectric encoder, as well as electronic equipment using the same, for detecting position, travel speed, travel direction and the like of a movable object by receiving, with a light receiving element, light transmitted or reflected by the movable object.
In printers, plotters, optical disk units and the like, photoelectric encoders are used in a mechanism for detecting displacement or displacement direction of a printing head or an optical head. As this type of photoelectric encoder, conventionally, there has been known a photoelectric rotary encoder disclosed in JP S59-40258 A.
FIG. 10 shows an optical part in the photoelectric rotary encoder disclosed in JP S59-40258 A. Referring to FIG. 10, a movable object 2 that travels along an arrow direction is placed between a light emitting section (not shown) and a light receiving section 1 which are placed opposite each other. In this movable object 2, a light passing zone (which normally has a slit form) PZ and a light non-passing zone NZ are formed alternately at a constant pitch P along the travel direction. The light receiving part 1 is so formed that four light receiving elements 1a, 1b, 1c, 1d each having a width ((¼)P) corresponding to one half of the width (½)P of the light passing zone PZ and the light non-passing zone NZ in the movable object 2 are arrayed without clearances.
Then, out of four signals outputted from the light receiving elements 1a, 1b, 1c, 1d (the signals denoted by A+, B−, A−, B+, correspondingly in order), the signal A+ and the signal A− are inputted to a comparator (not shown) so as to be compared with each other, while the signal B+ and the signal B− are inputted to another comparator (not shown) so as to be compared with each other. Then, two output signals different in phase from each other by 90° are obtained.
As for the material of the movable object 2 to be used for the photoelectric rotary encoder, normally, metals or synthetic resins are used. However, because of limitations of fine processing of these materials, their resolutions are also limited naturally. For example, when a conventional photoelectric encoder using a linear scale-type movable object 2 is applied to a printer, attaining a resolution of 300 dpi would cause the width (½)P of each of the light passing zone PZ and the light non-passing zone NZ to be about 40 μm. Still more, its allowance for machining process would be ±10 odd μm. As a result, it has conventionally been considered impossible to use the movable object 2 made of metal or synthetic resin for use in large-scale printers.
Further, when glass is used as the material of the movable object 2, it is true that such resolutions as described above are attainable. However, because glass is fragile and liable to breakage, there is a need for ensuring the mechanical strength, which leads to a problem of considerably increased cost.
This being the case, with a view to solving such resolution-related problems as described above, there has been proposed an optical encoder which uses a conventionally used movable object and which is capable of obtaining a resolution higher than that of the movable object, as in the case of an optical position encoder disclosed in JP 2604986 U.
FIG. 11 shows a placement relation between light passing zone PZ and light non-passing zone NZ regions of a movable object 6 and eight light receiving elements 5a-5h constituting a light receiving section 5 in the optical position encoder disclosed in JP 2604986 U, as well as a configuration of a signal processing section 7 for calculating output signals of the individual light receiving elements 5a-5h. 
Referring to FIG. 11, the optical position encoder includes a light emitting section (not shown), the movable object 6 in which the light passing zone PZ and the light non-passing zone NZ are formed alternately at a constant pitch P along its travel direction, a light receiving section 5 composed of eight light receiving elements 5a-5h arrayed on a straight line without clearances at a pitch of (⅛)P along the travel direction of the movable object 6, and a signal processing section 7 composed of eight adders 8a-8h, four comparators 9a-9d and two logical operation units 10a, 10b. 
FIG. 12 shows waveforms of output signals of the first comparator 9a through fourth comparator 9d as well as the first, second logical operation units 10a, 10b in the signal processing section 7. It is noted that the horizontal axis represents displacement of the movable object 6. Hereinbelow, operations of the signal processing section 7 are described with reference to FIG. 12.
FIGS. 12A, 12B, 12D and 12E show output signals SC1-SC4 of the first to fourth comparators 9a-9d, respectively. With respect to the output signal SC1, the output signal SC2 has a phase difference of −90°, the output signal SC3 has a phase difference of −45°, and the output signal SC4 has a phase difference of −135°.
The first logical operation unit 10a and the second logical operation unit 10b, as shown in FIG. 13A, are each composed of an AND element 11, a NOR element 12 and an OR element 13. As can he seen from a truth table shown in FIG. 13B, an output signal of H level is outputted if input signals are both at H or L level, and otherwise an output signal of L level is outputted. FIG. 12C shows an output signal OUT1 of the first logical operation unit 10a, and FIG. 12F shows an output signal OUT2 of the second logical operation unit 10b. 
As shown in FIGS. 12C and 12F, the first logical operation unit 10a and the second logical operation unit 10b produce output signals OUT1, OUT2, respectively, each having a 2-cycle waveform when the light passing zones PZ and the light non-passing zones NZ of the movable object 6 have traveled in the region of the light receiving section 5 to an extent of one pitch P. In this case, assuming that the one pitch P of repetition of the light passing zone PZ and the light non-passing zone NZ is 360°, then the output signal OUT2 of the second logical operation unit 10b is delayed in phase by 45° with respect to the output signal OUTS of the first logical operation unit 10a. That is, an output having a resolution two times higher than that of the movable object 2 having a phase difference of 90° can be obtained.
In this connection, because of limitations in fine processing of the light receiving elements 5a-5h in the above optical position encoder, resolutions higher than that of the movable object 6 can be obtained but attainable resolutions are limited.
For example, in order to attain an output of a 600 dpi resolution from the linear scale-type movable object 6 having a 300 dpi resolution, the array pitch of the light receiving elements 5a-5h becomes 10.6 μm. Like this, a theoretically required width of the light receiving elements 5a-5h is 10.6 μm at a maximum. However, in a case where there are no clearances between neighboring light receiving elements, because the light receiving elements tend to be more influenced by crosstalk, means (light insensitive material, light non-passing mask, etc.) for separating the neighboring light receiving elements from one another at a width of 5 μm to 10 μm need to be formed at boundary regions among the neighboring light receiving elements. As a result, the effective width of the light receiving elements 5a-5h to obtain a 600 dpi resolution output is 5.6 μm (10.6 μm-5 μm), given a 5 μm width of the “separation means”, resulting in 52.8% as a ratio of effective light receiving element width to the theoretically obtainable light receiving element width. Further, subdividing the width of the light receiving elements 5a-5h would involve higher-priced semiconductor processes.
Therefore, in the optical position encoder disclosed in JP 2604986 U, the more the light receiving elements 5a-5h are scaled down in width, the more the ratio of the effective light receiving element width to the theoretically obtainable light receiving element width decreases. As a result, the level of the output signals OUT1, OUT2 become smaller, causing the output signal to be easily influenced by crosstalk of signals of adjacent other channels, which may lead to problems of decreases in the S/N ratio of detection signals as well as increases in cost,